If you are lucky enough to have a decent local greengrocer, then try to foster a good relationship with them so that you are not only more likely to get the best of their produce but you can, if necessary, encourage them to source new and unusual leaves and veg for you.
Some of the supermarket chains have recently put a lot of effort into expanding their salad and vegetable lines. Many are now stocking some wonderful delights – even things like pea shoots (in lovely little ‘mini greenhouse’ packs) and peanut sprouts.